1. Field of the Invention
This invention relates to a radiation image read-out apparatus which reads out a radiation image stored on a stimulable phosphor sheet by the use of a line sensor.
2. Description of the Related Art
When certain kinds of phosphor are exposed to a radiation, they store a part of energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light or a laser beam, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as xe2x80x9ca stimulable phosphorxe2x80x9d. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as xe2x80x9cstimulated emission. There has been known a radiation image read-out apparatus in which a stimulating light beam such as a laser beam is caused to scan a stimulable phosphor sheet (a sheet provided with a layer of the stimulable phosphor) which has been exposed to a radiation passing through an object such as a human body to have a radiation image of the object stored on the stimulable phosphor sheet, the stimulated emission emitted from the stimulable phosphor sheet pixel by pixel is photoelectrically detected, thereby obtaining an image signal (a radiation image signal), and then the stimulable phosphor sheet is exposed to erasing light after the image signal is obtained from the stimulable phosphor sheet so that the residual energy of the radiation is fully released from the stimulable phosphor sheet. See, for instance, Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 55(1980)-116340 and 56(1981)-104645.
The radiation image signal thus obtained is subjected to image processing such as gradation processing and/or frequency processing and a radiation image of the object is reproduced as a visible image on the basis of the processed radiation image signal on a recording medium such as a photographic film or a display such as a CRT. When the stimulable phosphor sheet is exposed to erasing light, the residual energy of the radiation is fully released from the stimulable phosphor sheet and the stimulable phosphor sheet comes to be able to store a radiation image again, whereby the stimulable phosphor sheet can be repeatedly used.
In the radiation image read-out apparatus, a line light source which projects a line beam onto the stimulable phosphor sheet is used as a stimulating light source and a line sensor having a linear array of photoelectric convertor elements is used as a means for photoelectrically reading out the stimulated emission. The line beam is moved relative to the stimulable phosphor sheet and the line sensor in the direction perpendicular to the longitudinal direction of the line beam by a scanning means. The longitudinal direction of the line beam is referred to as xe2x80x9cthe main scanning directionxe2x80x9d and the direction perpendicular to the longitudinal direction of the line beam is referred to as xe2x80x9cthe sub-scanning directionxe2x80x9d. By the use of a line beam and a line sensor, the reading time is shortened, the overall size of the apparatus can be reduced and the cost can be reduced. See, for instance, Japanese Unexamined Patent Publication Nos. 60(1985)-111568, 60(1985)-236354, and 1(1989)-101540.
In the case of a transparent stimulable phosphor sheet 50, a stimulable phosphor layer 50a is supported by a support sheet 50b transparent to stimulated emission M and the line beam L is projected onto the stimulable layer side of the stimulable phosphor sheet 50 while the line sensor 21 is disposed on the support sheet side of the same to detect the stimulated emission M passing through the support sheet 50b as shown in FIG. 21. In such a transparent stimulable phosphor sheet 50, as well as in a reflective stimulable phosphor sheet where the line beam and the line sensor are disposed on the same side of the stimulable phosphor sheet, there is a problem that since the stimulating light L is scattered inside the sheet 50 and stimulating emission emitted from the phosphor layer 50a upon exposure to the stimulating light L is also scattered inside the sheet 50, the width dM of the stimulated emission M passing through the support sheet 50b becomes larger than the width dL of the line beam L.
As can be understood from FIG. 21, when the width dP of the line sensor 21 is smaller than the width dM of the stimulated emission M passing through the support sheet 50b, a substantial part of the stimulated emission M misses the line sensor 21, that is, the stimulated emission accumulating efficiency of the line sensor 21 is poor, and accordingly, a high quality image cannot be obtained.
It is important to minimize the amount of stimulated emission M which misses the line sensor in order to obtain a high quality image.
In Japanese Unexamined Patent Publication No. 2000-66316, we have proposed a radiation image read-out apparatus in which stimulated emission M emitted from a part of the stimulable phosphor sheet exposed to a line stimulating beams is detected by a line sensor comprising a two-dimensional array of photoelectric convertor elements (that is, the line sensor has a plurality of photoelectric convertor elements in both the main scanning direction and the sub-scanning direction) in which the sum of the widths of the photoelectric convertor elements in the sub-scanning direction is substantially equivalent to the width dM of the stimulated emission M passing through the support sheet 50b as measured on the plane of the light receiving face of the line sensor, and the output of each photoelectric convertor element for each scanning position is processed with respect to the portion of the stimulable phosphor sheet, thereby increasing the stimulated emission accumulating efficiency.
As shown in FIG. 22, the intensity of stimulated emission M is the most high at the portion corresponding to the width dL of the stimulating light beam and lowers outward. When a plurality of photoelectric convertor elements are arranged in the sub-scanning direction so that the sum of the widths of the photoelectric convertor elements in the sub-scanning direction is substantially equivalent to the width dM of the stimulated emission M passing through the support sheet as measured on the plane of the light receiving face of the line sensor, substantially the whole stimulated emission M can be accumulated. However, as light becomes weaker, noise becomes relatively stronger, and accordingly, noise becomes stronger relatively to the accumulated stimulated emission. From the viewpoint of cost, it is not preferred to arrange an excessive number of photoelectric convertor elements in the direction of width of the stimulated emission M.
In our Japanese Patent Application No. 2000-217516, there is disclosed a radiation image read-out apparatus using a detecting system comprising a plurality of line sensors arranged in the sub-scanning direction. In the patent application, the width of the detecting system is not mentioned to. When the width of the detecting system is substantially equivalent to the width dM of the stimulated emission M passing through the support sheet 50b as measured on the plane of the light receiving face of the line sensor, the same problem as in the radiation image read-out apparatus disclosed in Japanese Unexamined Patent Publication No. 2000-66316 arises.
In view of the foregoing observations and description, the primary object of the present invention is to provide a radiation image read-out apparatus in which the stimulated emission accumulating efficiency is improved and a high quality image is obtained without increasing noise and without substantially adding to the cost.
In accordance with the present invention, there is provided a radiation image read-out apparatus comprising a line stimulating light beam source which projects a line stimulating beam extending in a main scanning direction onto a stimulable phosphor sheet storing thereon radiation image information, a stimulated emission detecting means which receives stimulated emission emitted upon exposure to the line stimulating beam from line portions of the stimulable phosphor sheet exposed to the line stimulating beam and converts the amount of stimulated emission received to an electric signal, a sub-scanning means which moves the line stimulating beam and the combination of the line sensor and the stimulable phosphor sheet relatively to each other in a direction (sub-scanning direction) different from the main scanning direction, an image signal read-out means which reads out the output of each photoelectric convertor element in sequence at the respective positions at which the element is moved by the sub-scanning means, wherein the improvement comprises that the stimulated emission detecting means has a light receiving face whose width in the transverse direction of the line portion of the stimulable phosphor sheet exposed to the line stimulating beam is such that 30% to 90% of the amount of stimulated emission corresponding to a part of the stimulated emission spreading beyond the width of the line stimulating light beam as measured on the plane of the light receiving face of the stimulated emission detecting means can be received by the light receiving face in addition to the amount of stimulated emission corresponding to the width of the line stimulating light beam.
As the line stimulating light beam source, a fluorescent lamp, a cold cathode fluorescent lamp, an LED array and such may be employed. The line stimulating light beam source need not be like a line itself so long as it can emit a line stimulating light beam. For instance, a broad area laser may be employed. The stimulating light beam may be continuously emitted from the light beam source or may be emitted therefrom in a pulse-like fashion. From the viewpoint of reduction in noise, preferably the line stimulating light beam is in the form of high output pulsed light.
It is preferred that the length of the line stimulating light beam on the stimulable phosphor sheet be equivalent to or larger than the length of the side of the effective area of the stimulable phosphor sheet parallel to the line stimulating light beam.
It is suitable that the line stimulating light beam is 10 to 4000 xcexcm in width as measured on the surface of the stimulable phosphor sheet.
The direction in which the sub-scanning means moves the line stimulating beam and the combination of the line sensor and the stimulable phosphor sheet relatively to each other (will be referred to as xe2x80x9cthe sub-scanning directionxe2x80x9d, hereinbelow) is preferably a direction substantially perpendicular to the line stimulating light beam and the line sensor but may be any direction so long as substantially the entire surface of the stimulable phosphor sheet can be uniformly exposed to the stimulating light beam. Further, the sub-scanning means may move zigzag the line stimulating beam and the combination of the line sensor and the stimulable phosphor sheet relatively to each other.
In one embodiment of the present invention, as the stimulated emission detecting means, a line sensor 20 having a one-dimensional array of a number of photoelectric convertor elements 21 as shown in FIG. 23A is employed. In this case, the image signal read-out means reads out the output of each photoelectric convertor element in sequence in the respective positions to which the element is moved by the sub-scanning means. Each of the photoelectric convertor elements 21 has such a width Wa that 30% to 90% of the amount of stimulated emission corresponding to a part of the stimulated emission spreading beyond the width of the line stimulating light beam as measured on the plane of the light receiving face of the line sensor can be received in addition to the amount of stimulated emission corresponding to the width of the line stimulating light beam. It is preferred that the length of the line sensor on the stimulable phosphor sheet be equivalent to or larger than the length of the side of the effective area of the stimulable phosphor sheet parallel to the line stimulating light beam.
It is preferred that the size of each photoelectric convertor element in the main scanning direction be smaller than the size in the direction perpendicular to the main scanning direction.
In another embodiment of the present invention, as the stimulated emission detecting means, a line sensor 20xe2x80x2 having a two-dimensional array of photoelectric convertor elements 21xe2x80x2 as shown in FIG. 23B is employed. In this case, the image signal read-out means is provided with an operation means which carries out operational processing on the outputs of each photoelectric convertor element with respect to the portions of the stimulable phosphor sheet. In the line sensor 20xe2x80x2 shown in FIG. 23B, a plurality of photoelectric convertor elements are arranged in both the main scanning direction and the direction perpendicular thereto, and the sum Wb of the widths of the elements arranged in the direction perpendicular to the main scanning direction is such that 30% to 90% of the amount of stimulated emission corresponding to a part of the stimulated emission spreading beyond the width of the line stimulating light beam as measured on the plane of the light receiving face of the line sensor can be received in addition to the amount of stimulated emission corresponding to the width of the line stimulating light beam.
When the number of photoelectric convertor elements is large to such an extent that influence of transfer rate is recognizable, shortening of charge accumulating time due to increase in charge transfer time may be avoided by once storing the charge accumulated in each photoelectric convertor element in a memory, and reading out the charge from the memory during a next charge accumulating cycle.
It is preferred that the line sensor includes not less than 1000 photoelectric convertor elements in the longitudinal direction thereof, and that the light receiving face of the line sensor be not shorter than the corresponding side of the stimulable phosphor sheet.
Though the line sensor 20xe2x80x2 shown in FIG. 23B has a number of photoelectric convertor elements 21xe2x80x2 which are arranged in a straight line in each of the longitudinal and transverse directions of the stimulated emission but the photoelectric convertor elements 21xe2x80x2 may be arranged in other patterns. For example, the photoelectric convertor elements 21xe2x80x2 may be arranged zigzag in the transverse direction of the stimulated emission and arranged in a straight line in the longitudinal direction, and may be arranged zigzag in the longitudinal direction of the stimulated emission and arranged in a straight line in the transverse direction.
Further, the line sensor may comprise a plurality of sensor chips each comprising a plurality of photoelectric convertor elements. In this case, the chips maybe arranged in a straight line in the longitudinal direction of the line sensor or zigzag in the longitudinal direction of the line sensor. From the viewpoint of easiness of manufacture, it is preferred that each sensor chip comprises a plurality of photoelectric convertor elements arranged in both the longitudinal direction and the transverse direction.
In still another embodiment of the present invention, the stimulated emission detecting means comprises a plurality of line sensors arranged in the transverse direction. In this case, the image signal read-out means is provided with an operation means which carries out operational processing on the outputs of each line sensor with respect to the portions of the stimulable phosphor sheet. The line sensors may be disposed close to each other, or a part from each other. Further, the line sensors may be disposed even on opposite sides of the stimulable phosphor sheet.
FIG. 23C shows an example of the stimulated emission detecting means comprising a plurality of line sensors arranged in both the longitudinal direction and the transverse direction. That is, the stimulated emission detecting means shown in FIG. 23C comprises a pair of line sensors C1 and C2 which are arranged in the transverse direction are disposed above the stimulable phosphor sheet 50 and a line sensor C3 which is disposed below the stimulable phosphor sheet 50. The line sensor C1 comprises a number short line sensors (e.g., C1a, C1b, C1c), the line sensor C2 comprises a number short line sensors (e.g., C2a, C2b, C2c), and the line sensor C3 comprises a number short line sensors (e.g., C3a, C3b, C3c). The width of the stimulated emission detecting means is equivalent to the sum of the widths W1, W2 and W3 of the respective line sensors C1, C2 and C3, and is such that 30% to 90% of the amount of stimulated emission corresponding to a part of the stimulated emission spreading beyond the width of the line stimulating light beam as measured on the plane of the light receiving face of the stimulated emission detecting means can be received in addition to the amount of stimulated emission corresponding to the width of the line stimulating light beam.
AS the line sensor, an amorphous silicon sensor, a CCD sensor, a CCD sensor with a back illuminator, a MOS image sensor and such can be employed.
The operational processing which the image signal read-out means carries out on the outputs of the photoelectric convertor element may be simple addition, weighted addition or such. In such a case, the image signal read-out means may be an adder.
The expression xe2x80x9cthe amount of stimulated emission corresponding to a part of the stimulated emission spreading beyond the width of the line stimulating light beam as measured on the plane of the light receiving face of the line sensorxe2x80x9d means as follows. That is, the stimulated emission emitted from a portion of the stimulable phosphor sheet exposed to a line stimulating light beam of a first width generally spread and becomes wider than the first width as measured on the plane of the light receiving face of the line sensor. The width of the line stimulating light beam as measured on the stimulable phosphor sheet will be sometimes referred to asxe2x80x9cthe irradiation widthxe2x80x9d, hereinbelow. xe2x80x9cA part of the stimulated emission spreading beyond the width of the line stimulating light beam as measured on the plane of the light receiving face of the line sensorxe2x80x9d means the part by which the stimulated emission spreads beyond the width of the line stimulating light beam as measured on the plane of the light receiving face of the line sensorxe2x80x9d. When an enlargement/reduction optical system is inserted between the stimulable phosphor sheet and the line sensor, the width of the stimulated emission should be compared with the first width as enlarged or reduced by the enlargement/reduction optical system.
The irradiation width is a width of the line stimulating light beam as measured on the surface of the stimulable phosphor sheet. Ideally, the stimulating light beam does not diverge as shown by the broken line in FIG. 24 but actually somewhat diverges as shown by the solid line in FIG. 24. Accordingly, xe2x80x9cthe width of the line stimulating beamxe2x80x9d or xe2x80x9cthe irradiation widthxe2x80x9d is defined to be the distance between positions in which he intensity of the stimulating light beam becomes 1/e of the original intensity as denoted by dL in FIG. 24.
The hatched part in FIG. 25A shows the whole amount (100%) of stimulated emission corresponding to a part of the stimulated emission spreading beyond the irradiation width dL as measured on the plane of the light receiving face of the stimulated emission detecting means. The hatched part dmin in FIG. 25B shows 30% of stimulated emission corresponding to a part of the stimulated emission spreading beyond the irradiation width dL as measured on the plane of the light receiving face of the stimulated emission detecting means. The hatched part dmax in FIG. 25C shows 90% of stimulated emission corresponding to a part of the stimulated emission spreading beyond the irradiation width dL as measured on the plane of the light receiving face of the stimulated emission detecting means.
Spread of the stimulated emission is preferably measured in the following manner. That is, a stimulable phosphor sheet is exposed to the line stimulating light beam with the area to be exposed being limited by a lead plate and the stimulated emission is received by a CCD or photographic film. Then spread of the stimulated emission is measured in the term of the value of a count in the case of the CCD and in the term of the density distribution measured by a micro densitometer after development in the case of the photographic film.
It is preferred that the stimulable phosphor sheet be an anisotropic stimulable phosphor sheet which emits the stimulated emission in a direction inclined at a predetermined angle to the direction of thickness of the sheet.
It is further preferred that the anisotropic stimulable phosphor sheet be formed by anisotropic deposition.
It is preferred that a collector optical system for collecting the stimulated emission on the light receiving face of the stimulated emission detecting means be disposed between the stimulable phosphor sheet and the stimulated emission detecting means. As such a collector optical system, may be employed a refractive index profile type lens array such as a SELFOC(copyright) lens array which is formed by an imaging system where the object plane and the image plane are in one to one correspondence, a rod lens array and the like, a cylindrical lens, a slit, an optical fiber bundle or a combination of these optical elements be disposed between the stimulable phosphor sheet and the line sensor.
Further, the stimulated emission accumulating efficiency may be increased by bringing the stimulable phosphor sheet and the stimulated emission detecting means into a close contact with each other without inserting an imaging system. In such a case, an optical fiber bundle may be provided between the stimulable phosphor sheet and the stimulated emission detecting means to guide the stimulated emission from the stimulable phosphor sheet to the light receiving face of the stimulated emission detecting means.
It is further preferred that a stimulating light cut filter (a sharp cut filter, a band pass filter and such) which does not transmit the stimulating light but transmits the stimulated emission be provided between the stimulable phosphor sheet and the stimulated emission detecting means to prevent the stimulating light from entering the stimulated emission detecting means.
In accordance with the present invention, the stimulated emission accumulating efficiency can be moderately increased and a high quality image can be obtained without increasing noise and without substantially adding to the cost.
When a line sensor having a number of photoelectric convertor elements one-dimensionally arranged in the main scanning direction (the longitudinal direction of the stimulate emission) is employed as the stimulated emission detecting means, the apparatus can be simple in structure. In this case, when the size of each photoelectric convertor element in the longitudinal direction is smaller than that in the transverse direction, the stimulated emission accumulating efficiency can be increased without deteriorating the reading density (pixel density).
When a line sensor having a number of photoelectric convertor elements two-dimensionally arranged in the longitudinal direction and the transverse direction is employed as the stimulated emission detecting means, difficulties in making large size photoelectric convertor elements can be avoided since a large size photoelectric convertor element can be formed by a plurality of small size photoelectric convertor elements.
When the stimulable phosphor sheet is an anisotropic stimulable phosphor sheet which emits the stimulated emission in a direction inclined at a predetermined angle to the direction of thickness of the sheet, spread of stimulated emission itself is suppressed and accordingly the stimulated emission accumulating efficiency can be further increased.
When the anisotropic stimulable phosphor sheet is formed by anisotropic deposition, the purity of the stimulable phosphor can be increased and the stimulated emission emitting efficiency can be increased, which increases the stimulated emission accumulating efficiency. Further, the anisotropic stimulable phosphor sheet formed by anisotropic deposition is more easy to control as compared with anisotropic stimulable phosphor sheets formed by other methods.